Method of impregnating wood

ABSTRACT

A method of diffusion impregnation of wooden structures, which include long wooden objects, with a wood preservative, depots with the wood preservative being placed, in a way known per se, within the wood. The characteristic feature of the method is that in sections of the wooden object in which the moisture ratio of the wood is less than substantially 25 % depots with wood preservative dissolved in a hygroscopic liquid are placed and that in sections of the wooden object in which the moisture ratio of the wood is greater than substantially 25 % depots with wood preservative in solid phase are placed. The depots are placed at intermittent distances from each other along the wooden structure so that each long object is impregnated along the whole of its length and right through it.

The present invention refers to a method of impregnating woodenstructures which consist of long wooden objects, such as for examplewindow-frames, window-casements, window-posts, door-frames, joists,cappings, sills etc., with a wood preservative, depots or reservoirs ofthe wood preservative being placed within the wood in a way known perse.

The purpose of the wood preservative is to protect the wood againstbiological destruction. Diffusion impregnation (also called osmoticimpregnation) implies that the wood preservative diffuses into the moistwood. Diffusion impregnation is generally carried out by applying awater soluble wood preservative in the form of a paste or concentratedsolution to be surface of raw, undried timber. The preservative thendiffuses into the moist wood. By raw, undried timber is meant wood themoisture ratio of which exceeds about 25%. The moisture ratio is definedas the ratio of the mass of water in moist wood to the mass of the driedwood.

The British patent specification No. 912 381 describes diffusionimpregnation of the type mentioned in the introduction. However, themethod is suitable only for wood which is subjected to very moistconditions, for example wood in boats, quays, wooden structures buriedin earth etc.

The Swedish patent specification No. 7810771-1 also describes a methodof diffusion impregnation of the type described in the introduction. Thewood preservative is in the form of a fused body of boron oxide. Thisprior method is suitable for impregnating wood the moist ratio of whichis greater than about 25%. In wood the moisture ratio of which is lessthan about 25% the wood preservative spreads only to a very smallextent. Thus it is not possible, with a reasonable number of depots perunit of length, to impregnate a piece of wood the moisture ratio ofwhich is less than about 25%. The Swedish patent application No. 7803250describes a method of impregnation of the type described in theintroduction. The wood preservative is dissolved in a water-expellentliquid, usually an organic solvent. The water-expellent liquid expelsfree water and water bound in the wood so that the moisture ratio of thewood becomes a minimum of 21% (or more), which is a value that preventsthe growth of fungi. The method is suitable for use in wood the moistureratio of which is less than about 25%. In wood the moisture ratio ofwhich exceeds about 25% this prior method does not work. Nor can themethod be used for impregnating long pieces of wood in which themoisture ratio varies along the length of the wood piece. In a zone inwhich the moisture ratio of such a wood piece exceeds about 25% thewater is not expelled and the remaining water further prevents thespreading of the wood preservative used along the length of the woodpiece.

In the case of constructions of the type mentioned in the introductionthe moisture ratio varies heavily along the long wooden object. Themoisture ratio in the area of an end surface of the wooden objectexceeds 30% while further away from the end surface it may be less than20% and again further away possess moisture ratios which provide optimummoisture conditions of establishing fungus or insect attack.Accordingly, none of the methods described above is suitable for theimpregnation of a wood piece of such a nature. Either the impregnationis not able to penetrate into the relatively dry wood which will thusremain untreated or the relatively dry wood may be treated while therelatively moist wood remains untreated. In the method according to thesaid Swedish patent application also the relatively dry sections of thewood piece may theoretically be impregnated if one assumes that thedepots of the wood preservative are placed closely adjacent each other.In practice this would mean that the depots would have to be placed at adistance of the magnitude of 1 to 2 cm from each other so that the wholeof the dry volume of wood can be impregnated. However, this isnon-realistic since the wood piece will be perforated by bores. If thedistance between such depots is increased no such impregnation of theintermediate relatively dry sections takes place or such sections willbe impregnated only after a very long time, of the magnitude of severalyears.

According to another prior method of impregnation, which cannot becharacterized as a diffusion impregnation, the wood preservative isinjected under high pressure into the wood. When such injection takesplate in wood which is affected by fungus, there is a risk that the woodpreservative escapes from these affected areas by spurting out from thewood. Accordingly, the wood preservative will not spread further to thesound wood which surrounds the affected zones.

The present invention aims at achieving diffusion impregnation of longwooden objects along the whole of their length, also in those sectionswhere the moisture ratio is lower than substantially 25% and with theutilization of a reasonable number of depots per unit of length. In thewood treated by the method of the invention a retarding dose against thegrowth of fungus is achieved within a short period, of the magnitude ofabout 2 to 3 months, within a considerable range of spreading from thedepot.

The characteristic features of the invention are that in sections of thewooden object in which the moisture ratio of the wood is less thansubstantially about 25% depots or reservoirs of wood preservativedissolved in a hygroscopic liquid are placed, and that in sections ofthe wooden object in which the moisture ratio of the wood in greaterthan substantially about 25% depots of wood preservative in solid phaseare placed and that the depots are placed at intermittent distances fromeach other along the wooden structure so that each long wooden object isimpregnated along the whole of its length and right through it.

As wood preservative fluorine compositions or borate compositions areused the latter of which are preferred, and as hygroscopic liquid aglycol formula is used. The wood preservative in solid phase ispreferably a fused body of disodium metaborate or boron oxide in theshape of a cylindrical body. As a liquid impregnating agent disodiumtetraoctaborate dissolved in monoethylene glycol is preferably used.

A synergistic effect in respect of the spreading of the woodpreservative in the relatively dry wood is achieved with the woodpreservative dissolved in the hygroscopic liquid. The synergistic effectshows itself in the fact that the wood preservative spreads at a greatspeed in the relatively dry wood. The reason of this is not fullyunderstood but applicant believes that the hygroscopic liquid absorbsthe moisture existant in the wood (possibly also moisture from thesurroundings). Accordingly, the moisture ratio of the wood increases atthese places and the increased moisture ratio assists in the furtherspreading of the wood preservative through diffusion. Besides, thehygroscopic liquid spreads in the said wood with the increased moistureratio and functions as a carrier for the wood preservative so as therebyto assist in further spreading the wood preservative in the wood.

The body of fused biocide composition is dissolved by the moisture ratioof which exceeds 25 to 30% and spreads through diffusion to areas withinthe wood volume the moisture ratio of which is less than 25% where thedissolved impregnation agent remains and kills established fungus andpossible insect attacks.

Certain fungus living in wood has the capacity of spreading by itselftransporting the necessary water in its mycelium, whereby wood drierthan 25% can be infected and broken down.

By the present invention it is achieved that a hygroscopic and highlyviscous liquid, deposited within the volume of wood, remains in thevolume of wood which, concealed, may have broken-down, loosened timberand where dry zones have arisen but where remaining fungus myceliumwhich may be activated by remoistening is killed.

Besides, the advantage is achieved that the highly viscous woodpreservative spreads further into the sound wood which surrounds theaffected wood.

In the surrounding sound wood the wood preservative serves as apreventive protection against insect and fungus attack.

By the invention it is accordingly achieved that in dry zones of thewood there occurs an impregnation which prevents attack by insects andfungus, while in relatively moist zones the impregnation is curing, i.e.that established fungus and insect attack is killed otherwise theimpregnation constitutes a preventive protection in moist zones.

The depots are placed at intermittent distances from each other alongthe wooden structure so that a volume of the wood preservative in thesolid phase will give a lethal dose in a theoretic, spherical volume ofwood situated around the depot volume and being at least about 500 timeslarger than the volume of the wood preservative and so that a volume ofthe wood preservative in the liquid phase will give a lethal dose in atheoretic, spherical volume of wood situated around the depot volume andbeing at least about 100 times larger than the volume of the woodpreservative.

The invention will be described more in detail below in connection withthe attached drawings, in which

FIG. 1 is a front view of a window casement and the figure shows thefixed placings of the depots according to the invention substantiated byextensive tests,

FIG. 2 is a perspective view of a corner portion of a window casementand a window frame and the figure shows more closely details of thelocation of the depots fixed according to the invention,

FIG. 3 is a view similar to FIG. 1 showing the location of the depotsaccording to another embodiment of the invention,

FIG 4 is a view similar to FIG. 1 showing the location of the depots inthe case of a 4-pane window.

FIG. 5 is a diagram showing the moisture ratio at different distancesfrom a depot of wood preservative dissolved in a hygroscopic liquid,

FIG. 6 is a graph which shows the moisture ratio at different distancesfrom a depot in which a fused body of wood preservative has beeninserted,

FIG. 7 is a diagram which shows the content of wood preservative atdifferent distances from a depot in which there has been placed a woodpreservative in fused form,

FIG. 8 is a graph similar to FIG. 7 plotted for a depot located in newwood and containing wood preservative dissolved in a hygroscopic liquidand

FIG. 9 is a sectional view of a depot and closing means for the latter.

FIG. 1 shows a plane view of a window casement which has beenimpregnated in accordance with a first embodiment of the invention. Thewindow casement includes a casement sill member 1, side members 2 and 3,respectively, and a casement top member 4. According to this embodimentof the invention the window casement is impregnated only up to a heightof about 40 cm which in the normal case will give perfect impregnation.The casement sill member 1 is impregnated in the following way: a firstdepot or reservoir 5 is placed 3 to 8, preferably 5 to 8 cm, inward fromthe left-hand end surface, as seen in the figure, of the casement sillmember. The exact placing of this depot is dependent on the size of thepin, marked with dashed lines 6, of the casement side member 2. This pin6 is received in a corresponding recess, not shown, in the casement sillmember 1. The exact placing of the depot 5 is chosen so that it lies tothe right of the pin 6. The depot consists of a bore 7 (see FIG. 9)filled with wood preservative of a first kind, in the case fuseddisodium octaborate in the shape of a cylinder. The cylinder contains atleast 3 grams of boric acid per cubic centimeter. The bore is directedsubstantially at right angles to the plane of the window pane not shown.Preferably, the bore runs parallel to the exterior long bevel 8 (compareFIG. 2) of the casement sill member.

A second depot 9 is then placed at the opposite end of the casement sillmember 1 while observing the above rules. The depot is filled with thewood preservative of the first kind, marked with I in the drawing. Athird depot is then placed 10 to 15 cm beyond the depot 5 and a fourthdepot 11 is placed 10 to 15 cm beyond the depot 9. The depots 10 and 11have the same orientation as the earlier depots and are also filled withthe same wood preservative of the first kind. The remaining sections ofthe casement sill member are then divided up into regular stretches thelength of which is 15 to 25 cm and at the dividing points additionaldepots are placed each of which has the same orientation as the depots 5to 11 earlier mentioned but which are now filled with a woodpreservative of the second kind, namely disodium octaborate dissolved inmonoethylene glycol. 1.8 cm³ of the depot contents contains at least 0.3g boric acid, preferably 1.8 cm³. The wood preservative of the secondkind is sold in Sweden under the name of Boracol-40. In the drawingthese depots are marked with B. The wood preservative of the first kindis sold in Sweden under the name of IMPEL. In the drawing these depotsare marked with I.

Each casement side member 2, 3 is impregnated in the same way andtherefore only impregnation of the left-hand casement side member 2 isdescribed below. A first depot 12 filled with the wood preservative ofthe first kind is placed at a distance of 2 to 4 cm above the connectionwith the casement sill member 1. The depot is placed at substantiallyright angles to the plane of the window pane not shown. A second depot13 filled with wood preservative of the second kind is then placed 10 to15 cm above the first depot 12 in the casement side member. The depot isplaced parallel to the plane of the window pane not shown. A furthernumber of depots oriented in the same way as the depot 12 are thenplaced above the depot 13 at a regular mutual distance of from about 15cm to about 25 cm. These additional depots not shown in FIG. 1 arefilled with Boracol-40. In the normal case it is sufficient toimpregnate up to a height of about 40 cm of each casement side member 2,3.

If the window has a post 14 which has been marked with dashed lines inFIG. 1 the impregnation described in connection with the casement sillmember is modified by the measure that after the placing of the depot 11two bores 15, 16 are made on each side of the pin normally found in thepost and located in a corresponding recess in the casement sill member.The bores 15 and 16 are accordingly placed in association with the endsurfaces of said recess where the risk of injuries by moisture is great.To avoid shadow effect it is suitable to arrange a depot/bore on eachside of the pin. The bores 15, 16 are then filled with wood preservativeof the first kind. The two remaining sections of the casement sillmember 1 are then divided up in the way earlier described and as will beseen from FIG. 1 there will be only 3 depots which are filled with woodpreservative of the second kind, since the spacing of the depots 11 and16 is less than about 15 cm.

FIG. 2 shows a detail of a corner joint in connection with a window. Thewindow has an exterior window frame 18 and an exterior window pane 19.The sill member of the exterior window frame 18 is impregnated in thefollowing way: a first depot 20 filled with wood preservative of thefirst kind is placed 1 to 2 cm from the joining with the side member 21'of the window frame. A corresponding depot is placed at the other end,not shown, of the sill member. The remaining length between the twolastmentioned depots is divided up into regular spaces which are fromabout 15 to 30 cm and at the dividing points depots are placed which arefilled with wood preservative of the second kind. All the depots in thesill member are placed at right angles to the plane of the window pane19 (or are placed parallel to the plane of the pane and are applied frombelow). Each side member of the window frame 18 is impregnated in thesame way and therefore only the impregnation of the left-hand sidemember shown in FIG. 2 is described. In this a first depot 21 with woodpreservative of the first kind is placed approximately on a level withthe top surface of the sill member of the window frame. The depot 21stands at right angles to the plane of the window pane (or, if thedimension of the frame side member permits, can be oriented parallel tothe plane of the window pane). A second depot 23 is then placed 10 to 15cm above the first depot 21 and is directed parallel to the plane of thewindow pane 19. This depot is shown only diagrammatically in FIG. 2. Itis seen that in the practising of the invention the window is opened andthereupon a hole is bored in the side member towards the left-handsurface of the casement side member 2. Then further depots are placed ata mutual distance of about 15 to about 30 cm from each other above thesaid second depot 23 up to a height of about 40 cm.

FIG. 3 shows another embodiment of the method according to theinvention, the window having been impregnated all around which isnecessary under certain circumstances, for example if the window isseriously decayed. The location of the depots at the top of the windowis reversed in relation to that at the bottom of the window andtherefore need not be described more closely. The locations areindicated diagrammatically in the figure and I designates woodpreservative of the first kind while B designates wood preservative ofthe second kind. FIG. 4 shows a 4-pane window, i.e. a window which hasfour window pane units capable of being opened. The window frames notshown are impregnated in the same way as the window frame according tothe FIGS. 2 and 3 while the location of the depots in the centre post 24and the transverse posts 25, 26 is seen in the figure. The basicprinciple is that a depot of wood preservative of the first kind isplaced in close association with an end surface which is exposed tomoisture. Further away from such an end surface depots may be placedwhich are filled with wood preservative of the second kind.

FIG. 5 shows the effect of the wood preservative in increasing themoisture ratio. In a long piece of wood the moisture ratio of whichinitially was about 10% depots filled with wood preservative of thesecond kind were placed. After about 4.5 months the piece of wood wassawn up into slabs parallel to the depot and the moisture ratio wasmeasured in the various wooden slabs. As will be seen from the graph themoisture ratio has risen to about 18% and thereabove in areas withinabout 12 cm from the depot. A corresponding graph of moisture ratio(FIG. 6) was plotted for depots with wood preservative of the firstkind. Here the wood originally had a moisture ratio of about 30%. As inthe previous case the wood in this case was new. From the graph it isseen that the moisture ratio had risen to about 50% within an area of 8cm around the depot after about 4.5 months.

FIG. 7 is a graph which shows the spreading of boric acid from a depotof wood preservative of the first kind placed in a window which isinstalled in a building face. The depot is supposed to be located inorigo and the abscissa shows the distance in cm from the depot while theordinate shows the number of kilograms of boric acid per cubic meterabsorbed by the wood. The moisture ratio at the depositing of the woodpreservative was initially greater than 25%. The dashed graph shows theamount absorbed after about 3.5 months and the full line graph shows thecorresponding amount after 9 months. The retarding dose for the growthof fungus is, for the wood preservative used, converted to theequivalent amount of boric acid, 1.5 kg boric acid per cubic meter. Thelethal dose is 6 kg per cubic meter. From the figure it is seen thatafter 9 months a retarding dose is found at a distance of up to 12 cmfrom the depot while a lethal dose occurs up to a distance of 6 cm fromthe depot.

FIG. 8 is two graphs similar to FIG. 7, one of which, in full line,shows the spreading of boric acid from a depot of wood preservative ofthe second kind placed in dry wood (moisture ratio 22%) and the other ofwhich (the one in dashed line) shows the corresponding spreading from adepot of wood preservative of the second kind placed in raw wood(moisture ratio 50%). As will be seen the retarding dose is found at thedistances 4.5 and 6.5 cm, respectively, out from the respective depotafter a time of 2.5 months.

Tests carried out by Sveriges Lantbruksuniversitet, The Institution ofWood Technology, show that in the case of using borate formulas as afungicide the retarding dose varies for different fungi between 0.8 and1.2 kg boric acid equivalent/cubic meter of wood. As a qualifiedretarding dose of boric acid absorption of 1.5 kg boric acidequivalents/cubic meter of wood is recommended and this dose has to bereached within 9 months from the deposition of the preservative. As arelatively quick-acting lethal dose the absorption of 6 kg boric acidequivalent/cubic meter of wood is recommended. As a slow-acting lethaldose absorption of 2 to 3 kg boric acid/cubic meter is recommended whichkills fungi. A cylinder of fused boron octaborate with the diameter 8.5mm and the length 10 mm has the volume 0.57 cubic centimeter andcontains 1.71 g boric acid which theoretically is sufficient toimpregnate 285 cubic centrimeters of wood with a lethal dose. From thisit can accordingly be established that a cubic centrimeter of the fusedcartridge with a lethal dose theoretically can impregnate 500 cubiccentrimeter of wood. The corresponding figure for a retarding dose is2000 cubic centimeter of wood. One cubic centimeter Boracol 40 contains0.575 g boric acid which theoretically is sufficient to impregnate about100 cubic centrimeters of wood with a lethal dose. The correspondingfigure for impregnating with a retarding dose is about 400 cubiccentimeters of wood.

Table 1 shows the maximum spreading of boric acid of the first kind andthe second kind, respectively, in sound wood analyzed with a colourreagent. The figures stated refer to spreading from one side of thedepot. The total spreading consequently is double as much. The spreadingwas measured after a period of

    ______________________________________                                        2 months                                                                      Impel   Spreading 1 cm  U = 22%    (1 test)                                   Boracol Spreading 7 cm  U = 20%    (1 test)                                   Impel   Spreading 11 to 13 cm                                                                         U = 44%    (1 test)                                   Boracol 15 to 17 cm     U = 50%    (1 test)                                   and                                                                           4 months                                                                      Impel   Spreading 1 cm  >U = 23%   (4 tests)                                  Boracol 11 cm           >U = 23%   (4 tests)                                  Impel   Spreading >20 cm in sapwood, 3 to 11 in                               heartwood           U = >40%   (4 tests)                                      Boracol >20 cm in sapwood, 9 to 13 in                                         heartwood           U = >40%   (4 tests)                                      ______________________________________                                    

The table shows that wood preservative of the first kind spreads verybadly in relatively dry wood while it spreads far, more than 11 cm, inrelatively moist wood. In sapwood it spreads more than 20 cm if themoisture ratio of the wood is greater than 40%. The table also showsthat wood preservative of the second kind spreads far in relatively drywood, more exactly more than 11 cm within 4 months.

FIG. 9, in the end, shows a section of a depot in the form of a bore 7in which a cylinder 27 of fused disodium octaborate is inserted. Thedepot is closed with a cylindrical sleeve 28 having annular skirts orbarbs 29 projecting from the wall and made integral with the sleeve. Thesleeve has a bottom wall 30 which is made integral with the rest of thesleeve. Centrally in the said bottom there is made a through hole 31. Bymeans of a solid lid 32 the open end of the sleeve may be closed.

The working procedure in impregnating a window is preferably thefollowing: first bores 7 are bored at the places of the depots as aboveindicated. In the depots to be filled with wood preservative of thefirst kind the solid cylinders 27 are inserted. Then sleeves 28 areinserted into all the holes bored, and in the bores to be filled withwood preservative of the second kind the wood preservative is introducedfor example through a hose or a thick cannula which is inserted in thesleeve 28 through the opening 31. The prescribed number of cubiccentimeters of the liquid phase is dosed. All the sleeves 28 are theneach closed by a lid 32. The liquid in the depots which are filled withwood preservative of the second kind disappears gradually out into thewood and no further refilling of these holes is of current interestsince the amount introduced is intended to give the dose which is neededto establish a preventive protection against fungus growth in therelatively dry sections of the wooden piece. In the bores in which woodpreservative of the first kind has been introduced the cylinders remainthere during a considerable time, of the order of tens of years, and thecylinders are dissolved and spread the boric acid when the moistureratio in the surrounding wood exceeds about 30%. By removing the lid 32and inserting a stick or the like through the hole 31 serving as aninspection opening it may be established if the cylinder 27 remainsthere or not. Depots filled with such cylinders may possibly need to berenewed after the said time by depositing a new cylinder.

The embodiments of the invention described above may be modified andvaried in many different ways within the scope of the basic idea of theinvention.

I claim:
 1. A method of impregnating a wooden structure by diffusion ofboric acid, said wooden structure of the type including a long woodenobject along the length of which a moisture ratio varies, said methodcomprising the steps of:drilling bores along the length of said woodenstructure at intermittent distances from each other; inserting inselected ones of said bores disodium octaborate in the form of a solidfused body, whereby said disodium octaborate dissolves under theinfluence of moisture in said wood, to react and form boric acid, whichdiffuses into said wood; inserting in remaining ones of said boresdisodium octaborate dissolved in a hygroscopic liquid, whereby saiddisodium octaborate migrates together with said hygroscopic liquid intosaid wood, and reacts with water bound in cellulose to form boric acid;said steps of inserting being further characterized by selecting anumber of bores with fused bodies relative to the number of bores withliquid, such that the total amount of boric acid formed constitutes afungus inhibitory dose and being substantially a fungus lethal dose fora volume of wood in said wooden structure; said step of drilling saidbores being further characterized by the step of: selecting theintermittent distances between bores such that said boric acidpenetrates said entire wooden structure, and sealing said bores afterthe insertion of said boric acid.
 2. A method according to claim 1,characterized in that the hygroscopic liquid is a glycol composition. 3.A method according to claim 2, characterized in that said bores areseparated by intermittent distances such that said boric acid releasedfrom said solid fused body of disodium octaborate inserted in one ofsaid bores will impregnate a wood volume which is at least 2000 timesthe volume of said body, and such that said disodium octaboratedissolved in said hygroscopic liquid will release boric acid in anamount that will impregnate a wood volume which is at least 400 timesthe volume of said bore in which it is inserted.
 4. A method ofdiffusion impregnation according to claim 3, said wooden structureincluding a sill, characterized by the steps of placing a solid fusedbody of disodium octaborate in a bore about 3 to 8 cm inward from oneend surface of said sill, placing a second solid fused body in a bore ata corresponding distance from an opposite end surface of said sill,placing a third solid fused body in a bore at a distance of about 10 to30 cm from the first bore, placing a fourth solid fused body in a boreat a distance of about 10 to 30 cm from said second bore, and placingadditional portions of disodium octaborate dissolved in hygroscopicliquid in bores at predetermined intervals between said solid fusedbodies in said third and fourth bores, said predetermined intervalsbeing about 15 to 25 cm, and further characterized by the step ofsealing said bores.
 5. A method of diffusion impregnation according toclaim 4, wherein there is at least one mullion extending vertically fromsaid sill, characterized by the step of placing a solid fused body ofdisodium octaborate in a bore on either side of a pin of the mullionprojecting into said sill.
 6. A method of diffusion impregnationaccording to claim 4, the wooden structure including a jamb,characterized by the step of placing a solid fused body of disodiumoctaborate in a first bore about 3 cm above a joint between said jamband said sill, placing an amount of disodium octaborate dissolved insaid hygroscopic liquid in a second bore 10 to 15 cm above said firstbore, and placing additional amounts of disodium octaborate dissolved insaid hygroscopic liquid 10 to 20 cm above said second bore up to aheight of about 40 cm above said sill.
 7. A method of diffusionimpregnation according to claim 6, wherein said jamb is impregnatedalong its length, characterized by the steps of drilling a first boreabout 3 cm above said joint, drilling a second bore 10 to 15 cm abovesaid first, bore drilling a third bore about 3 cm below a joint existingbetween a window frame head and said jamb, drilling a fourth bore about20 to 30 cm below the third bore, drilling additional bores atpredetermined intervals to each other and to said second and fourthbores, said predetermined interval being about 15 to 30 cm, filling saidfirst and third bores with solid, fused bodies of disodium octaborate,and filling said additional bores with an amount of disodium octaboratedissolved in said hygroscopic liquid.
 8. A method of diffusionimpregnation according to claim 7, characterized by the step of fillingsaid second and fourth bores with solid, fused bodies of disodiumoctaborate.
 9. A method of diffusion impregnation according to claim 6,said wooden structure including a mullion, characterized by the step ofimpregnating said mullion in the same manner as said jamb.
 10. A methodof diffusion impregnation of a wooden structure including a casementhaving a sash with top and bottom rails and two stiles, characterized bythe steps of placing a solid, fused body of disodium octaborate in abore 1 to 2 cm from either end of said bottom rail of said sash, andplacing at either end of either stile an amount of disodium octaboratedissolved in hygroscopic liquid in a bore in alignment with the upperedge of said bottom rail of said sash and the lower edge of said toprail of said sash.
 11. A method of diffusion impregnation according toclaim 10, further characterized by the step of placing a solid, fusedbody of disodium octaborate in a bore 1 to 2 cm from either end of saidtop rail of said sash.